feat: Add Nix development shell config to use (additional to containerized hooks)

[gabyx]

It is extremely helpful if we could use Nix devShells to run stuff.
The functionality would be the same as the one for images.yaml which defines images which can be used

Hook Run Configuration Version 4

cmd: ...
args: ...
env: ...
image: ...
nix: 
  path: ./tools/nix
  name: "myshell"
  
version: 4 # optional

Then nix develop <base-path>/tools/nix#myshell will be used, <base-path> is relative the shared repo or the current executing one.

There should be an Git option githooks.nixHooksEnabled which will take precedence over githooks.containerizedHooksEnabled and will run over the nix develop ... --comand shell.